1. Field of the Invention
This invention relates to a process for extruding a synthetic resin and to a die for use in practising the method.
2. Description of the Prior Art
In extruding generally a synthetic resin sheet or film, it is believed necessary to smooth the resin flow path surface as much as possible in order to obtain a sheet product having a smooth surface and to prevent deterioration of quality due to stagnation of the resin on a resin flow path surface of the extruding die.
In the conventional die having the resin flow path surface smoothed in this manner, local slip of a resin tends to occur on the resin flow path surface of the die if a resin which is extremely slippery with respect to the surface such as an oil-containing polyacetal resin, for example, is extruded into the sheet form. In such a case, the velocity distribution of the resin flow becomes non-uniform at the outlet of the die and a portion having an extremely varying flow velocity within a narrow distance in the direction of width of the die, or a local flow, takes place. This local flow cannot be corrected by such means as adjustment of a choke bar or lip opening.
The local flow arises from the fact that the resin does not slip uniformly over the resin flow path surface of the die. As means for preventing the local flow, therefore, there may be employed such methods as coating of a lubricant or TEFLON in advance onto the resin flow path surface of the die in order to render the resin flow path surface more slippery. The more slippery the resin flow path surface was, however, the more vigorous became the local flow, in accordance with our experiments.
If a sheet-like article is extrusion formed by cooling and withdrawing while the local flow occurs, the resulting sheet product has non-uniformity in its thickness over its widthwise direction; hence, it is impossible to obtain a quality product. For this reason, it has been conventionally believed that a resin extremely slippery relative to the resin flow path surface such as oil-containing resins as exemplified by an oil-containing polyacetal resin, can not be used for extrusion.